It is known to use metals selected from the group containing gold, silver and copper to provide the actual active layers. These layers are arranged between an adhesive layer and a protective layer, and the adhesive layer may likewise possess optical properties and can consist, for example, of a metal such as chromium or of a dielectric. Dielectric material is normally used for the protective layer (DE-AS No. 2029181 and DE-AS No. 2334152). Such layers suffer from the disadvantage of flecking, which varies in intensity depending upon the metal used for the active layer and which, within the available range, can only be avoided or modified with difficulty. Marked changes in colour characteristic lead to pronounced variations in the reflection and tranmission behaviour, since these optical properties are closely linked with the colour characteristic of the glazing.
It is also known to dispense with a purely metallic layer and to produce colour-neutral layers which reflect a high infra-red proportion of the radiation. This is achieved by means of a first layer of indium-tin oxide which is reinforced by a second layer consisting of at least one of the oxides of tin, bismuth, titanium and tantalum. However, the transmission values are above 70%, and furthermore the colour impressions cannot be intentionally varied (DE-OS No. 32 01 783).